My brother just found out he is HIV positive. His girlfriend wants to have a child with him but shes not infected. Is this even possible?

Yes, but..... It is possible for her to get pregnant with him, but it is also likely he will pass the infection to her....And the baby.
Yes. Absoluetly it is. Its the actual semen which contains the HIV virus not the sperm itself. So..The sperm can be washed by a lab. The data is quite good. Inform your brother that taking his HIV medication is important too (becoming undetectable) because it really dramatically drops the percentage chance to infect another.